1. Field of the Invention
The present invention relates to a fluorescent lamp, and more particularly to a method and an apparatus for manufacturing a flat fluorescent lamp.
2. Description of the Related Art
A fluorescent lamp, particularly a fluorescent lamp is in wide use for manufacturing a backlight unit of a liquid crystal device (LCD). Generally, the fluorescent lamp has various shapes including a straight shape, a serpentine shape, a flat shape, and so forth. Glass is molded into the various shapes of the lamp at high temperature to form discharge channels inside the lamp.
The inside surfaces of the discharge channels is coated with a fluorescent material and remains in vacuum. The discharge channels are kept in vacuum, employing an exhaust process. The exhaust process is performed at high temperature, for example 400° C., in a furnace (now shown) to remove impurities such moisture and humidity existing in the discharge channels. After finishing the exhaust process, an inert gas and a mercury vapor are supplied to the discharge channels and thereafter the discharge channels are hermetically sealed.
Thereafter, a mercury vapor diffusion process is performed in which the supplied mercury vapor is uniformly distributed within the discharge channels. The flat fluorescent lamp, unlike a bar-shaped lamp in use at home and work, has a configuration in which the bar-shaped discharge channel with a small diameter is formed over a long distance like a tunnel, or a plurality of the bar-shaped discharge channels are connected to each other through a narrow passage formed in between. The mercury vapor is diffused and distributed through the narrow passage from the channel to its neighbors. This makes it difficult to make a uniform distribution of the mercury vapor within the channels. The mercury vapor diffusion process, therefore, is critical in manufacturing the flat fluorescent lamp. The mercury vapor diffusion process of applying a heat treatment to the fluorescent lamp at about 250° C. is performed to uniformly distribute the mercury vapor within the channels
The failure to uniformly distribute the mercury vapor within the discharge channels requires more time in a subsequent aging process, thus lengthening a manufacturing time for the fluorescent lamp.
A cold-cathode-tube-typed fluorescent lamp needs to go through the aging process, as the last process for manufacturing the fluorescent lamp, for more than one hour. The aging process, by which discharge occurs within the discharge channels by supplying an electrical current to external electrodes on both of the ends of the fluorescent lamp, is performed to maintain a constant value of electrical current at the first time of lighting up the fluorescent lamp.
As shown in FIG. 1, the fluorescent lamp is practically exposed to an atmosphere and therefore cools down at time intervals between the exhaustion process, the mercury diffusion process, and the aging process.
FIG. 2A is a table of an a test result illustrating a relationship between a defect percentage and a mercury diffusion time necessary for diffusion of a mercury vapor into the fluorescent lamp manufactured with a conventional method. A heat-treatment temperature for the diffusion of the mercury vapor was set to 250° C. during the test. A fluorescent lamp, which needed an increase of 10% or higher in terms of a reference driving voltage when lightened up about 12 hours after finishing the aging process, was defined as defective one. A lack of a heat-treatment time necessary for the diffusion of the mercury vapor into the channels may cause the mercury vapor to concentrate upon certain region without being distributed uniformly over entire regions within the discharge channel. This is known as “a pink charge phenomenon,” because a mercury-vapor-concentrated region turns pink color when discharge occurs. The pink charge phenomenon results in increasing the driving voltage after finishing the aging process. It is very difficult to detect the critical defect such as the pink charge phenomenon in advance. One of ways to reduce the defect is to perform the mercury vapor diffusion process for 5 hours or more. On the other hand, a fluorescent lamp for an LCD TV should be enabled to be lighted up at a low temperature. However, the increase in the driving voltage due to the defect prevents the fluorescent lamp from being lighted up at the low temperature.
FIG. 2B is a table of another test result illustrating the relationship between the defect percentage and the time for the mercury vapor diffusion necessary for the diffusion of the mercury vapor into the fluorescent lamp manufactured with the conventional method. The heat-treatment time for the diffusion of the mercury vapor was set to one hour during the test. The table indicates that the diffusion of the mercury vapor was in smooth progress above a temperature of 356° C. at which the mercury exists in the gaseous phase and that the defect percentage remarkably decreased, compared to the defect percentage which was observed below a temperature of 356° C. However, the defects still occurred by 5 percentage points above the temperature of 356° C.
The mercury melts and freezes at temperatures of 356° C. and −39° C., respectively. The mercury exists in the liquid phase at room temperature. The mercury has vapor pressure of about 0.002 mmHg at room temperature, about 0.28 mmHg at 100° C., and about 79 mmHg at 250° C., respectively. The characteristics of the mercury, when temperatures are not uniform in the discharge channels during the mercury vapor diffusion process, causes the mercury vapor to be condensed around the region where a temperature is relatively low, and therefore increases mercury density around the mercury-vapor-condensed region, compared to that of the other region. This prevents the mercury vapor from being uniformly distributed within the fluorescent lamp and therefore prevents the mercury vapor from uniformly emitting light, thus lengthening the aging time in the subsequent aging process. This also causes shortage of mercury vapor around some region within the discharge channels as time goes by after lighting up the fluorescent lamp, thus shortening a lifetime of the fluorescent lamp. According to a conventional method for manufacturing the flat fluorescent lamp, a gas inlet through which the inert gas and the mercury vapor are supplied protrudes from a surface of the flat fluorescent lamp at a right angle to the surface of the flat fluorescent lamp. The protruding gas inlet requires the whole thickness of the back light unit to be larger to protect against the breakage of the gas inlet when combining the fluorescent lamp with the backlight unit. Furthermore, the protruding gas inlet of the flat fluorescent lamp should be kept in the upright position, when air is exhausted from the inside of the flat fluorescent lamp through the gas inlet to keep the inside of the flat fluorescent lamp in vacuum and when the inert gas and the mercury vapor are supplied through the gas inlet. The upright position of the protruding gas inlet requires more occupying space for operation and therefore decreases operating efficiencies.
According to another conventional method for manufacturing the flat fluorescent lamp, an exhaust pipe protrudes from any of sides and surfaces of the flat fluorescent lamp. The property of glass to expand in all directions due to high temperature in the furnace during the exhaustion process prevents the exhaust pipe, which is made of the glass, from maintaining an original position of the exhaust pipe and therefore causes the exhaust pipe to suffer from breakage.